The invention relates to a process for the separation of methanol and methylmethacrylate (MMA).
Methanol is used in the manufacture of MMA. One process for making MMA involves the oxidative esterification of methacrolein (MAL) with methanol. In the manufacturing process, mixtures of MMA and methanol are formed. It is difficult to separate the components from these mixtures, as MMA and methanol form an azeotrope or a “near azeotrope,” also referred to as a “tangent pinch.” This means that the components cannot be separated by conventional distillation, and that special measures are needed to accomplish the separation.
JP 03819419 B2 describes a methanol recovery column where the methanol and methacrolein are separated from MMA in a distillation column with no other separating agents added. The overhead composition is limited by the azeotropic composition (11 wt % of MMA in methanol). While the azeotropic composition can be approached by using a large number of trays and/or a high reflux ratio, the MMA composition in the overheads cannot be less than the azeotropic composition. This is undesirable as the MMA is the desired product, and sending it back to the reactor requires larger equipment and, more importantly, provides the opportunity for the valuable product to react further to by-products, thereby lowering the MMA yield.
U.S. Pat. No. 4,518,462 describes the removal of methanol from MMA using a C6-C7 saturated hydrocarbon, e.g., hexane, cyclohexane, heptane, methyl cyclopentane or dimethylpentane, as an entrainer. No water is added to the overheads decanter, so the phases split into hydrocarbon-rich and methanol-rich layers. One of the drawbacks of this approach is the limited ability to dry the recycle stream. In addition, in order to reduce the MMA to low levels in the recycle stream, a large amount of entrainer is required, resulting in high energy usage and a large and expensive distillation column.
U.S. Pat. Nos. 5,028,735, 5,435,892, and JP 02582127 B2 describe a similar entrainer process where either sufficient water is in the feed or water is added to the overhead decanter to form an organic and aqueous layer. In this case, essentially all of the hydrocarbon entrainer resides in the organic layer. The aqueous layer can be sent to a drying column to remove water from the recycle stream; however, large amounts of hexane are still required to minimize MMA in the recycle stream. For example, U.S. Pat. No. 5,028,735 describes an entrainer process using hexane as the entrainer with hexane usage of at least 17-fold the water content of the feed and 3-fold the methanol in the feed.
U.S. Pat. No. 6,680,405, uses methacrolein as an entrainer. While the azeotrope composition was broken, it resulted in only a minor improvement, namely 7.4% MMA in the recycle stream.
In view of the deficiencies of the prior art, it would be desirable to have an improved method for the separation of methanol and MMA.